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I'm trying to wrap my head around NFC tag inner workings. In particular I'm curious whether different coils are used for harvesting energy and to transmit data. This random NFC tag IC datasheet I found has separate pins for power and antenna, which makes me think that there are at least two coils involved. However, wont the two coils interfere with each other given their proximity? Does this imply that NFC tags have different stages of accumulating energy and sending data, or is it happening simultaneously?

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  • \$\begingroup\$ That chip only has one antenna connection (AC0/AC1 being both ends). The V_EH I think you are referring to is an output to provide energy harvested from the antenna to external regulators which can then be used to power the VCC/user interface if not self-powered. \$\endgroup\$ Sep 4, 2023 at 8:42
  • \$\begingroup\$ No, i understand that much. But the VCC implies there is another coil since it's a tag and is meant to use induced current? Or does it have just a single coil both used for power and as an antenna? \$\endgroup\$ Sep 4, 2023 at 8:45

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NFC tags have exactly one antenna and your IC also has only one set of pins for an antenna. To my understanding the different standards on which NFC tags are based on do not allow the use of more than one antenna. In ISO/IEC 14443 and 15693, for example, there are clearly defined antenna classes which "only" contain one antenna. Both data transfer and power transfer must occur over the same antenna.

Regardless of this, there is always an influence on energy harvesting during data transmission. The energy harvesting (rectification) happens permanently, but the amount depends on the ongoing communication state. For example: When the tag transmits, it shorts the antenna (depending on the standard), which means that no energy can be received. This means there must always be enough energy stored to ensure that the communication is not interrupted.

Now two edge cases can occur. There may not be enough energy for communication (e.g. due to a too long distance). Therefore, many tag ICs have the possibility to be powered externally (the VCC pin on your IC). Quite often, however, the other case occurs, meaning there is more than enough energy available. Your tag IC allows the excess energy to be used by providing an unregulated DC output voltage at pin VEH (which is a special feature).

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NFC tags usually have one coil for harvesting energy and transmitting data (simultaneously). The tag ICs may have seperate power pins for applications where e.g. the tag needs to be powered even when there is no reader present.

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